The clinical use of lasers has escalated in the last 20 years, in particular for treating diseases and disorders of the eye. Different wavelengths of laser light have been used to seal leaky retinal blood vessels, to remove debris from the posterior capsule of the lens after cataract surgery, and to correct refractive errors by reshaping the corneal stroma. The latter has proved to be an increasingly popular option for correction of refractive error compared to other means of refractive error correction such as contact lenses and spectacles. In addition to treating uniform refractive errors (myopia, hyperopia, astigmatism) laser vision correction technology has been used to treat corneal surface irregularities such as scars and various corneal dystrophies. This treatment of irregular corneal surface disorders is referred to as phototherapeutic keratectomy (PTK) and involves ablating a protruding region of cornea into a smoother surface.
In order to produce a uniform corneal surface by PTK the lower regions surrounding the protruding areas of cornea must be filled with a suitable medium, otherwise a uniform laser beam will simply ablate the current pattern further into the cornea. To provide such a medium, a masking fluid of high absorption (and therefore small penetration depth) and moderate viscosity is applied to the cornea, filling in the “valleys” and allowing uniform ablation of the cornea by the laser beam. This results in smoothing out of the irregular corneal surface. Masking fluids for laser refractive surgery using excimer lasers include balanced salt solution (BSS) and 0.9% saline solution.
Commercial laser refraction correction techniques have generally relied on the ultraviolet wavelength range and these have typically been developed using an excimer laser, which has been the mainstay of laser refractive surgery for several years. The excimer laser requires an argon-fluoride gas mixture as the optical media for the oscillation chamber. Although successful refractive error corrections have been made using the excimer laser, there are some problems that are associated with the use of gas as an optical medium. In addition, the gas requires continual replacement, which results in additional cost as well as a requirement for storage facilities.
These problems have been removed with the development of solid-state lasers, which use crystals as optical media instead of gas. Due to the different optical media used, there is a different wavelength emitted from the solid state laser (213 nm) compared to the excimer laser (193 nm). As a result, a larger penetration depth is potentially exhibited by the solid-state laser beam. As proposed in applicant's international patent publication WO 01/58398, this increased penetration depth is potentially advantageous as problems associated with excess fluid on the corneal surface during excimer laser surgery may be avoided when using a solid state laser. However, studies have shown that the same masking fluids used for excimer laser corrective surgery show markedly less light absorption when used for the same techniques using solid state lasers. This implies that these masking fluids are not suitable for PTK and another masking fluid is required if this procedure is to be executed using a solid-state laser.